Size mattered for flying dinosaurs: New research into pterosaurs and pelycosaurs shows their grand headcrests and sails were developed to attract a mate, not to regulate body temperature as first thought.

Now, you may think me a palaeontological pedant (& this is not a cue for cracks about my age from the cheap seats!), but I find that loose use of the word ‘dinosaur’ really irritating. Why? – because neither pterosaurs nor pelycosaurs were dinosaurs. I know the headline’s more eye-catching than saying ‘size mattered for prehistoric flying reptiles’, but still, it bugs me.

So what’s the difference?

Pelycosaurs – animals like Dimetrodon & Edaphosaurus - pre-dated the dinosaurs by a considerable length of time. They first appeared in the Carboniferous period (around 300 million years ago – here’s a link to a geological time scale) & were the dominant animal group during the Permian (280-260 mya). So what, you say. They could have been ancestral to dinosaurs. Well, no, they weren’t – & the evidence for this lies in their skulls. These splay-legged reptiles, some of which sported great ‘sails’ on their backs, were synapsids – a term reflecting the presence of a single opening (behind the eye) in the dermal bone of the skull. (Have a look at the picture at that link.) Mammals are also synapsids, & this means that the mammalian lineage has its roots much further back in time than that of the dinosaurs, who first appeared on the scence in the Triassic. What’s more, dinosaurs are diapsids – a feature that they share with birds & other living reptiles. (Which is why modern phylogenetic trees see birds branching off the reptiles rather than having a branch of their own, which is the way I was taught it way back when.)

The name ‘pterosaur’ means ‘winged lizard’, but these prehistoric flying reptiles are only distantly related to modern lizards. These elegant fossilsfirst turn up in rocks from the Triassic, but their complexity suggests that the group is probably older than that; it’s just that the fossils haven’t been found. Scientists used to think that pterosaurs were gliders, but the consensus has shifted to an acceptance that most (with the exception of the really big ones such as Quetzocoatlus) were capable of powered flight. (The evidence for this includes the fact that the bones of pterosaurs were hollow – like those of birds – and the forelimb bones had prominent muscle-attachment crests, suggesting the presence of flight muscles.) There’s also a suggestion, based on what looks to be fur on the bodies & flight membranes of some particularly well-preserved fossils, that at least some pterosaurs were endotherms ie capable of generating & maintaining their own body heat.

As for the dinosaurs, which evolved during the Triassic (around 225 mya)… I remember that Stephen Jay Gould once commented – as a throwaway line in one of his books – that people have a fascination with dinosaurs made all the more delicious by the fact that they were very fierce, very big – & very dead. (I wish I could remember the actual book I read that in; no time right now to go through my bookcase…) Of course, he would also have noted that this fascination is based on a misunderstanding of dinosaurs: they ranged in size from the giant sauropods like Diplodocus & the even larger titanosaurs, through the smaller but still large-by-human-standards tyrranosaurs, down to animals roughly the size of a turkey. I strongly suspect they’d have varied in ferocity as well :-)

If asked how to distinguish between dinosaurs & other diapsids, a palaeontologist would look at things like the anatomy of the legs. The earliest dinosaurs were bipeds, walking erect on their hind legs. In other words, the early reconstructions of some dinosaurs that showed them as quadrupeds with legs akimbo were a long way from reality. They walked on their toes, with the soles of their feet raised off the ground, and the hind legs in particular were pulled in under the body – essentially the same as in modern birds. In addition, the hip socket had a small opening in it where the 3 bones that make up each side of the pelvis come together. (Features of the hip also allow us to divide dinosaurs into 2 main groups – the ‘lizard-hipped’ saurischians & the ‘bird-hipped’ ornithischians. The latter is something of a misnomer as birds are actually most closely related to a group of saurischians.)

So – dinosaurs, pterosaurs & pelycosaurs are 3 related but distinct groups; conflating them into one ignores all their fascinating differences & oversimplifies our view of the past.

(I’d like to be able to comment on the actual paper on which that headline was based. Unfortunately, while we supposedly have an institutional subscription, at present the website keeps asking me for $US, so until that’s sorted out a review will have to wait.)

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